1. Studies of the mechanisms for the induction of in vivo tumor immunity: T cells isolated from FBL-3 ascites growth were highly cytotoxic in vitro but lacked long lasting in vivo anti-tumor immunity. Further studies showed that the lack of in vivo protective effect of these T cells was attributed to the presence of suppressor macrophages which were able to suppress the in vivo protective effect of immune T cells against tumor challenge. After removal of the macrophages, then the in vivo protective effect of the T cells from progressor tumors became readily demonstrable. 2. Regulation of T cell-mediated immunity by prostaglandins and antigens: Two immunoregulatory suppressor circuits are involved in the generation of cytotoxic T lymphocyte (CTL) response. 1) In the absence of antigen, endogenous production of prostaglandins regulates the activation of cytotoxic lymphocyte precursors and prevents the "spontaneous activation" of the cytotoxic precursors. 1) During antigen sensitization, both antigen-specific and antigen-nonspecific suppressor T cells are generated. The antigen specific suppressor T cells help to determine the magnitude of CTL response and the antigen-nonspecific suppressor T cells help to determine the specificity of CTL response. 3. Regulation by lymphokines of the cell mediated immunity: Initial endogenous production of Interleukin 2 (IL2) is essential for the differentiation and proliferation of cytotoxic T lymphocyte precursors into CTL. The production of higher levels of IL2 at a later time induces the generation of antigen-nonspecific suppressor T cells and augments the generation of antigen specific suppressor T cells. These suppressor T cells help to determine the specificity and magnitude of CTL response. 4. Generation of lymphokine-induced cytotoxic cells (LICC): LICC are generated by culturing normal spleen cells with Interleukin 2 (a T cell product) and a novel lymphokine, the cytotoxic cell differentiation factor (a macrophage product). The LICC selectively kill tumor cells in vitro and also possess strong in vivo anti-tumor activity.